U.S. patent number 7,693,490 [Application Number 11/690,468] was granted by the patent office on 2010-04-06 for multi-equalization method and apparatus.
This patent grant is currently assigned to Ecole de Technologie Superieure Polyvalor, Limited Partnership. Invention is credited to Mohamed Lassaad Ammari, Philippe Dumais, Francois Gagnon, Yvon Savaria, Claude Thibeault.
United States Patent |
7,693,490 |
Gagnon , et al. |
April 6, 2010 |
Multi-equalization method and apparatus
Abstract
A method and apparatus is disclosed for performing a
multi-equalization of a transmitted signal on a channel having
varying characteristics comprising equalizing the transmitted
signal using a plurality of setting defining a plurality of
equalizing functions to provide a corresponding plurality of symbol
signal, synchronizing each of the plurality of symbol signals to
provide a plurality of synchronized signals, selecting at least one
of the plurality of synchronized signals according to at least one
transmission performance criterion and providing the selected one
of the plurality of synchronized signals.
Inventors: |
Gagnon; Francois (Lachine,
CA), Savaria; Yvon (Montreal, CA), Dumais;
Philippe (Montreal, CA), Ammari; Mohamed Lassaad
(Montreal, CA), Thibeault; Claude (Brossard,
CA) |
Assignee: |
Ecole de Technologie Superieure
Polyvalor, Limited Partnership (Montreal, Quebec,
CA)
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Family
ID: |
36089819 |
Appl.
No.: |
11/690,468 |
Filed: |
March 23, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070293147 A1 |
Dec 20, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/CA2005/001465 |
Sep 26, 2005 |
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60612513 |
Sep 24, 2004 |
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Current U.S.
Class: |
455/65;
455/3.06 |
Current CPC
Class: |
H04B
7/005 (20130101); H04B 3/14 (20130101); H04L
25/03006 (20130101); H04L 2025/03726 (20130101); H04L
2025/03535 (20130101) |
Current International
Class: |
H04B
1/00 (20060101) |
Field of
Search: |
;455/65,3.06 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
International Search Report dated Jan. 17, 2006 from the
international application PCT/CA2005/001465. cited by other .
Dumais et al., "Multi-Equalization a Powerful Adaptative Filtering
for Time Varying Wireless Channels", Department of Electrical
Engineering, Ecole de Technologie Superieure, Montreal, Canada,
Fall 2004. cited by other.
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Primary Examiner: Anderson; Matthew D
Assistant Examiner: Bilodeau; David
Attorney, Agent or Firm: Ogilvy Renault LLP
Claims
We claim:
1. A multi-equalizer unit for improving signal transmission of a
signal transmitted on a channel having varying characteristics,
said multi-equalizer unit comprising: at least two equalizers, each
for equalizing said transmitted signal to provide a corresponding
plurality of symbol signals; a synchronization unit for
synchronizing each of said corresponding plurality of symbol
signals to produce therefrom a plurality of synchronized signals;
and a decision unit for selecting at least one of said plurality of
synchronized signals according to at least one transmission
performance criterion, corresponding to a synchronized mean square
error (MSE) calculated from one of said at least two
equalizers.
2. The multi-equalizer as claimed in claim 1, wherein said decision
unit is further for selecting said at least one transmission
performance criterion.
3. The multi-equalizer as claimed in claim 2, wherein said at least
two equalizers are further for calculating, for each of said
corresponding plurality of symbol signals, a corresponding mean
square error (MSE) over a number symbols of said corresponding
plurality of symbol signals, further wherein said decision unit is
further for using each one of said corresponding MSE for selecting
said at least one transmission performance criterion.
4. The multi-equalizer as claimed in claim 3, wherein said
synchronization unit is further for synchronizing each of said
corresponding MSE, further wherein said decision unit is further
for using each one of said synchronized corresponding MSE for
selecting said at least one transmission performance criterion.
5. The multi-equalizer as claimed in claim 3, wherein said
synchronization unit comprises a unit for performing
synchronization of said symbols.
6. The multi-equalizer as claimed in claim 1, wherein said at least
two equalizers comprises a frequency linear transversal equalizer
(FLTE) with frequency block least mean square (FBLMS) adaptation, a
decision-feedback equalizer (DFE) and a recursive least-square
(RLS) algorithm-based equalizer.
7. A method for improving signal transmission of a signal
transmitted on a channel having varying characteristics, said
method comprising: equalizing said transmitted signal using a
plurality of settings defining a plurality of equalizing functions
to provide a corresponding plurality of symbol signals;
synchronizing each of said corresponding plurality of symbol
signals to produce therefrom a plurality of synchronized signals;
selecting at least one of the plurality of synchronized signals
according to at least one transmission performance criterion,
corresponding to a synchronized mean square error (MSE) calculated
from one of said at least two equalizers; and providing said
selected one of said plurality of synchronized signals.
8. The method as claimed in claim 7, further comprising selecting
said at least one transmission performance criterion.
9. The method as claimed in claim 8, further comprising
calculating, for each of said corresponding plurality of symbol
signals, a corresponding mean square error (MSE) over a number
symbols of said corresponding plurality of symbol signals, and
further comprising using each one of said corresponding MSE for
selecting said at least one transmission performance criterion.
10. The method as claimed in claim 9, further comprising
synchronizing each of said corresponding MSE, and further
comprising using each one of said synchronized corresponding MSE
for selecting said at least one transmission performance
criterion.
11. The method as claimed in claim 9, further comprising performing
synchronization of said symbols.
12. The method as claimed in claim 7, wherein said plurality of
equalization functions comprises a frequency linear transversal
equalizer (FLTE) function with frequency block least mean square
(FBLMS) adaptation, a decision-feedback equalizer (DFE) function
and a recursive least-square (RLS) algorithm-based equalizer
function.
13. A software defined radio comprising a receiver for receiving a
signal transmitted on a channel having varying characteristics, and
a multi-equalizer unit for improving signal transmission of said
transmitted signal, said multi-equalizer unit comprising: at least
two equalizers, each for equalizing said transmitted signal to
provide a corresponding plurality of symbol signals; a
synchronization unit for synchronizing each of said corresponding
plurality of symbol signals to produce therefrom a plurality of
synchronized signals; and a decision unit for selecting at least
one of said plurality of synchronized signals according to at least
one transmission performance criterion, corresponding to a
synchronized mean square error (MSE) calculated from one of said at
least two equalizers.
14. The software defined radio as claimed in claim 13, wherein said
decision unit is further for selecting said at least one
transmission performance criterion.
15. The software defined radio as claimed in claim 14, wherein said
at least two equalizers are further for calculating, for each of
said corresponding plurality of symbol signals, a corresponding
mean square error (MSE) over a number symbols of said corresponding
plurality of symbol signals, further wherein said decision unit is
further for using each one of said corresponding MSE for selecting
said at least one transmission performance criterion.
16. The software defined radio as claimed in claim 15, wherein said
synchronization unit is further for synchronizing each of said
corresponding MSE, further wherein said decision unit is further
for using each one of said synchronized corresponding MSE for
selecting said at least one transmission performance criterion.
17. The software defined radio as claimed in claim 15, wherein said
synchronization unit comprises a unit for performing
synchronization of said symbols.
18. The software defined radio as claimed in claim 13, wherein said
at least two equalizers comprises a frequency linear transversal
equalizer (FLTE) with frequency block least mean square (FBLMS)
adaptation, a decision-feedback equalizer (DFE) and a recursive
least-square (RLS) algorithm-based equalizer.
Description
TECHNICAL FIELD
This invention relates to the field of telecommunications. More
precisely, this invention pertains to the field of equalizers.
BACKGROUND OF THE INVENTION
Channel equalization is known to be a powerful technique to reduce
intersymbol interferences (ISI) caused by multipath propagation
phenomenons as well as amplitude or phase distortions. For several
years, the use of equalizers led to high numerical complexity
filters. However the evolution of microelectronics offers today the
possibility to design much more complex equalizers at a relatively
low cost in terms of power or silicon surface consumption.
High quality filters are needed for wireless communication systems
which are used over a wide range of link configurations. In these
communication systems, channel equalization consists of an adaptive
filtering algorithm.
Unfortunately, choosing the optimal filtering structure depends on
channel characteristics that are often unknown a priori and time
varying. Even if channel characteristics are known or estimated, it
is very difficult to select the optimal equalizer and to predict
the converging properties or the channel tracking capabilities of
any equalizer.
There is a need for a method and apparatus that will overcome the
above-identified drawbacks.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and apparatus
for equalizing a transmitted signal in the case where channel
characteristics change.
According to a first aspect of the invention, there is provided a
multi-equalizer unit for improving signal transmission of a signal
transmitted on a channel having varying characteristics, the
multi-equalizer unit comprising at least two equalizers, each
receiving the transmitted signal, equalizing and providing a
corresponding plurality of symbol signals, a synchronization unit
receiving each of the corresponding plurality of symbol signals and
providing a plurality of synchronized signals and a decision unit
receiving the plurality of synchronized signals and selecting at
least one of the synchronized signals according to at least one
transmission performance criterion.
According to another aspect of the invention, there is provided a
method for improving signal transmission of a signal transmitted on
a channel having varying characteristics, the method comprising
equalizing the transmitted signal using a plurality of setting
defining a plurality of equalizing functions to provide a
corresponding plurality of symbol signals, synchronizing each of
the plurality of symbol signals to provide a plurality of
synchronized signals, selecting at least one of the plurality of
synchronized signals according to at least one transmission
performance criterion and providing said selected one of the
plurality of synchronized signals.
According to another aspect of the invention, there is provided a
software defined radio comprising a receiver for receiving a signal
transmitted on a channel having varying characteristics, and a
multi-equalizer unit for improving signal transmission of the
transmitted signal. The multi-equalizer unit is the one described
above.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the present invention will
become apparent from the following detailed description, taken in
combination with the appended drawings, in which:
FIG. 1 is a block diagram showing a multi-equalizer unit according
to an embodiment wherein n equalizers are used;
FIG. 2 is a flowchart showing how a multi-equalizer unit operates
according to an embodiment;
FIG. 3 is a flowchart showing how a synchronized output signal is
selected using at least one transmission performance criterion;
FIG. 4 is a block diagram showing an embodiment of a
multi-equalizer unit which comprises three equalizers.
It will be noted that throughout the appended drawings, like
features are identified by like reference numerals.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a multi-equalizer unit 6 according to an
embodiment.
The multi-equalizer unit 6 comprises a plurality of equalizer units
8, a synchronization unit 18 and a decision unit 20.
More precisely, the plurality of equalizer units 8 comprises a
first equalization unit 10, a second equalization unit 12, a
n-1.sup.th equalization unit 14 and a n.sup.th equalization unit
16. It will be appreciated that n is larger or equal to 2.
Each of the plurality of equalizer units 8 receives a transmitted
signal and performs an equalization of the transmitted signal
according to a corresponding setting defining an equalizing
function.
The first equalization unit 10 receives the transmitted signal and
provides a first equalized signal comprising a first given
plurality of symbol signals, while the second equalization unit 12
receives the transmitted signal and provides a second equalized
signal comprising a second given plurality of symbol signals. The
n-1.sup.th equalization unit 14 receives the transmitted signal and
provides a n-1.sup.th equalized signal comprising a n-1.sup.th
given plurality of symbol signals and the n.sup.th equalization
unit 16 receives the transmitted signal and provides a n.sup.th
equalized signal comprising a n.sup.th given plurality of symbol
signals.
It will be appreciated that each of the plurality of equalizer
units 8 needs to be complementary in order to increase the global
efficiency of the multi-equalizer unit 6.
The skilled addressee will further appreciate that in order to
design a wireless communication link, many channel parameters have
to be taken into account. The fluctuations of the delay profile
represents an important factor to be considered. When channel
echoes are long, an equalizer must have a long impulse response.
Such an equalizer converges slowly and involves an increase in the
computation complexity. Furthermore, long equalizers are not
efficient when fast fading occurs.
The synchronization unit 18 receives the first equalized signal,
the second equalized signal, the n-1.sup.th equalized signal and
the n.sup.th equalized signal and provides a plurality of
corresponding synchronized signals. In fact, a time synchronization
of the received signals is performed by the synchronizing unit
18.
The decision unit 20 receives each of the corresponding
synchronized signals and at least one transmission performance
criterion signal and provides an output signal corresponding to at
least one of the corresponding synchronized signals matching the at
least one transmission performance criterion signal. In an
embodiment, more than one transmission performance criterion signal
may be used. Also, in an embodiment, it is possible to use a
combination of the corresponding synchronized signals in order to
provide a more reliable decision. One way of doing this for
instance consists of using the most probable transmitted symbol
detected for each of the plurality of equalizers 8, and producing
global data using a majority, or a weighted majority algorithm.
Now referring to FIG. 2, there is shown how a multi-equalizer
operates according to an embodiment.
According to step 30 a transmitted signal is provided to a
plurality of equalizers. It will be appreciated that the
transmitted signal has been transmitted on a channel having varying
characteristics. Each of the equalizers equalizes the received
signal using a setting defining an equalizing function.
According to step 32, each output signal of the equalizers is
synchronized in time.
According to step 34, a synchronized output signal is selected
using at least one transmission performance criterion. As mentioned
previously, more than one transmission performance criterion may be
used.
Now referring to FIG. 3, there is shown how a synchronized output
signal is selected.
According to step 40, at least one transmission performance
criterion is provided.
According to step 42, each synchronized output signal of the
equalizers is provided.
According to step 44, the at least one transmission performance
criterion is applied to each output of the equalizers in order to
select an output signal. Alternatively, it will be appreciated that
a combination of at least one of the output signals of the
equalizers may be selected using the at least one transmission
performance criterion, as explained above, to provide a more
reliable decision.
Now referring back to FIG. 2 and according to step 36, the selected
synchronized output signal is provided.
Now referring to FIG. 4, there is shown an example of an embodiment
of the multi-equalizer unit 6.
The multi-equalizer unit 6 comprises the plurality of equalizer
units 8, the synchronization unit 18 and the decision unit 20.
The plurality of equalizer units 8 comprises a first equalizer 50,
a second equalizer 52 and a third equalizer 54.
In this embodiment, the first equalizer 50 has been designed for
long echoes. However, when an equalizer is designed in the time
domain, the computational complexity increases linearly with the
length of the filter. By using Fast Fourier Transform (FFT), it is
possible to implement it in the frequency domain. This reduces the
number of computations of the algorithm. Radix-2 butterfly FFTs can
be considered in order to reduce the complexity. Since the temporal
convolution is a multiplication in the frequency domain, the
complexity increases logarithmically and permits the implementation
of much longer equalizers. For example, with 32 taps, the
implementation in frequency is half of the time domain complexity.
A frequency linear transversal equalizer (FLTE) with frequency
block least mean-square (FBLMS) adaptation with 256 taps for the
first equalizer 50 works well. It has been contemplated that longer
length could also be used, but this length being about at least ten
times greater than a typical time domain equalizer is well suited
to demonstrate the usefulness of the complete architecture. With
Radix-2 FFTs, the complexity is also ten times less than the
equivalent (i.e. same length) linear transversal equalizer (LTE)
with an LMS algorithm.
In this embodiment, the second equalizer 52 comprises a shorter
decision-feedback equalizer (DFE). Generally, the length of the
feedback filter is small. 32 forward taps and 4 feedback taps work
well. The complexity of the second equalizer 52 therefore stays
amenable. The second equalizer 52 is therefore well suited for
shorter impulse response. The convergence of the decision-feedback
equalizer (DFE) is also much faster than the convergence of the
frequency linear transversal equalizer (FLTE) with frequency block
least mean-square (FBLMS). The skilled addressee will appreciate
however that the IIR structure may cause a divergence of such a
decision-feedback equalizer (DFE).
In this embodiment, the third equalizer 54 comprises a 16-tap
recursive least-square (RLS) algorithm. Such a linear equalizer
uses a non-linear adaptation algorithm which is different from LMS
algorithm of the previous equalizers which use Least Mean Square
adaptation. In fact, the Recursive Least Square algorithm is a
recursive algorithm which exploits accumulated statistics in order
to optimize the convergence of the filter taps. The equalizer is
useful when channel conditions vary rapidly in time. Consequently,
the RLS impulse response cannot be long.
It will be appreciated that the decision unit 20 operates according
to a transmission performance criterion which is based on the
smallest mean square error (MSE) of the output of each of the
plurality of equalizer units 8. More precisely, at any time k, the
output of the equalizer unit having the smallest mean square error
is chosen. In this embodiment, the mean square error value is
obtained by averaging the error over a limited but significant
number of symbols. More precisely, the mean square error value
MSE.sub.k.sup.n of an equalizer n is equal to
.lamda.MSE.sub.k.sup.n+(1-.lamda.)|{circumflex over
(x)}.sub.k-{tilde over (y)}.sub.k.sup.n|.sup.2, where {x.sub.k} is
a modulated symbol stream signal to be transmitted over the
multi-path channel, {y.sub.k} is the transmitted signal received at
the multi-path equalizer unit 6 (therefore also sometimes referred
to as the "received signal"), {{tilde over (y)}.sub.k.sup.n} is the
output of equalizer n, {{circumflex over (x)}.sub.k} is an
estimated value of the transmitted data. The mean square error
value may be computed either with a training sequence or with an
estimated value of the transmitted data {{circumflex over
(x)}.sub.k}. In one embodiment .lamda. has been chosen equal to
0.99 in order to obtain an average on the last 100 error values. It
will be appreciated that the computation of the mean square error
value is computed by each of the plurality of equalizer units
8.
It should be understood by the skilled addressee that the decision
unit 20 may operate according to various embodiments based on
distance between an equalized signal and a reference. The reference
may be generated using at least one of the usual detected symbols,
a statistic of the transmitted signal (such as the average
amplitude or a set of higher order moments) and a known training
sequence. Furthermore, it should be appreciated that even the mean
square error may be computed differently by averaging over disjoint
blocks of data or by using a sliding window algorithm for
instance.
Moreover, various other transmission performance criteria may be
used. For instance, a transmission performance criteria may be
provided using channel information provided by a channel estimation
unit. In such case, it would be pertinent to favor the use of an
equalizer in particular depending on the channel information. Bit
Error Rate (BER) data may also be used to generate a transmission
performance criterion when a training sequence is used.
The first equalizer 50 therefore provides a first mean square error
value signal 70 and a first equalized signal 80.
The second equalizer 52 provides a second mean square error value
signal 72 and a second equalized signal 82.
The third equalizer 54 provides a third mean square error value
signal 74 and a third equalized signal 84.
The synchronization unit 18 comprises a first synchronization unit
56 and a second synchronization unit 58.
The first synchronization unit 56 receives the first equalized
signal 80, the second equalized signal 82 and the third equalized
signal 84, performs a time synchronization of the signals and
provides a corresponding first synchronized signal 92, a
corresponding second synchronized signal 94 and a corresponding
third synchronized signal 96.
The second synchronization unit 58 receives the first mean square
error value signal 70, the second mean square error value signal 72
and the third mean square error value signal 74 and performs a time
synchronization of the received signals to provide a corresponding
first synchronized mean square error value signal 86, a
corresponding second synchronized mean square error value signal 88
and a corresponding third synchronized mean square error value
signal 90.
The decision unit 20 comprises a multiplexing unit 60 and a
selecting device 62.
The selecting device 62 receives the first synchronized mean square
error value signal 86, the second synchronized mean square error
value signal 88 and the third synchronized mean square error value
signal 90 and provides an equalizer selection signal 98
representative of the equalizer which has the lowest corresponding
synchronized mean square error value. In an embodiment, the
equalizer selection signal 98 is and index that is provided to the
multiplexing unit 60.
The multiplexing unit 60 receives the first synchronized signal 92,
the second synchronized signal 94 and the third synchronized signal
96 and provides one of the received signals according to the
equalizer selection signal 98. In the embodiment shown in FIG. 4,
the selected index is used for determining which one from the
corresponding first synchronized signal 92, the corresponding
second synchronized signal 94, and corresponding third synchronized
signal 96 will be selected as the output to the multi-equalizer
unit 6.
It will be appreciated by the skilled addressee that while some
example of equalizer units have been disclosed, other types of
equalizer units may be used for the plurality of equalizer
units.
It will be appreciated that the multi-equalizer unit 6 may be
advantageously used in a software defined radio (not shown). The
software defined radio would have, in addition to the
multi-equalizer unit, a receiver for receiving a signal transmitted
on a channel having varying characteristics (i.e., {v.sub.k}).
In such a case, the multi-equalizer unit 6 may be implemented in at
least one of a Field Programmable Gate Array (FPGA), a Digital
Signal Processing Unit (DSP), a Central Processing Unit (CPU), an
Application Specific Integrated Circuit (ASIC), Complex
Programmable Logic Device (CPLD) or the like. Moreover, it will be
appreciated that the plurality of equalizer units 8 may be provided
according to various parameters. Also, in an embodiment, at least
one of the plurality of equalizer units 8 may be updated/amended
depending on channel conditions. The at least one transmission
performance criterion may also be updated depending on a specific
use.
While illustrated in the block diagrams as groups of discrete
components communicating with each other via distinct data signal
connections, it will be understood by those skilled in the art that
the preferred embodiments are provided by a combination of hardware
and software components, with some components being implemented by
a given function or operation of a hardware or software system, and
many of the data paths illustrated being implemented by data
communication within a computer application or operating system.
The structure illustrated is thus provided for efficiency of
teaching the present preferred embodiment.
It should be noted that the present invention can be carried out as
a method, can be embodied in a system, a computer readable medium
or an electrical or electro-magnetical signal.
The embodiments of the invention described above are intended to be
exemplary only. The scope of the invention is therefore intended to
be limited solely by the scope of the appended claims.
* * * * *